In recent years, a vehicle that has a power storage device (e.g., a secondary battery, a capacitor and the like) mounted thereon and runs using driving force generated from electric power stored in the power storage device has received attention as an environmentally-friendly vehicle. This vehicle includes, for example, an electric vehicle, a hybrid vehicle, a fuel cell vehicle and the like.
In the above-mentioned vehicle, an inverter is generally used to convert DC electric power provided from the power storage device into AC electric power for driving a rotating electric machine such as a motor generator. Then, driving force generated by the rotating electric machine is used to cause the vehicle to run. During regenerative braking, rotational force provided from a driving wheel, an engine and the like is converted into electrical energy to charge the power storage device.
From the viewpoints of densification of field magnetic flux, ease of electric power regeneration and the like, a permanent magnet-type synchronous motor in which a permanent magnet is embedded in a rotor is used in some cases as the above-mentioned rotating electric machine mounted on the vehicle. Generally, the permanent magnet is known to change its properties depending on the ambient temperature. For example, in low ambient temperature, the magnetic flux density of the permanent magnet increases. As a result, at the time of low temperature, the counter-electromotive voltage generated by rotation of the rotating electric machine increases.
Japanese Patent Laying-Open No. 2008-043094 (PTL 1) discloses the technique of, in a vehicle including a motor generator having a rotor provided with a permanent magnet, determining a direct axis (d axis) current target value such that the direct axis current target value changes temporally, as well as setting a horizontal axis (q axis) current target value to zero and raising the temperature of the permanent magnet with the vehicle maintained at a stopped state, when the temperature of the permanent magnet is lower than a prescribed temperature at the start of operation.